It has nothing to do with real kings. It's about youth. When you are young, you rule the world. No one can help but love the optimism and strength of youth, but then, you realize that it isn't honest and it isn't going to last. You become jaded and aged like all those before. "Oh and who would ever want to be king?" As in, who would ever want to remain young once you realize how unstable and full of dreams not reality youth is. And in the end, he knows St. Peter will call his name--he feels older, wiser, and justified. That's how I see it.